The present invention relates to a method of electrophotographically producing a visual toner image on a recording sheet, and more particularly to an electrophotographic image-forming method by which a toner image having a high density, high resolution and therefore high quality can be produced on a recording sheet without formation of fogging, generation of undesired by-products such as ozone due to corona-discharging in charging section and/or transferring section, etc.
It is known that electrophotographic processes and electrostatic recording or printing methods are generally used to duplicate or reproduce an analog or digital data such as characters, graphics, etc. For instance, in an electrophotographic copying apparatus or facsimile, an electrostatic charge is produced on a photosensitive layer or dielectric layer and exposed to an image light to form a electrostatic latent image thereon. The latent image is developed by contacting a toner to form a visual toner image. Generally, the toners used there are those which are electrostatically charged to a proper polarity by a frictional contact with carrier particles according to a magnetic brush method, or magnetic toners which are composed primarily of a magnetic powder and a resin binder. However, in case such an electrophotographic image-forming method is adopted, the electrophotographic copying apparatus absolutely necessitates an electrostatic latent image-forming means including charging unit in addition to a light-illuminating means and a developing means to homogeneously charge a surface of an image-bearing member, resulting in complicated structure, large-scale equipment, etc.
U.S. Pat. No. 3,816,840 discloses a method in which a magnetic conductive toner is magnetically attracted to and retained on a sleeve made of a non-magnetic conductive material and a recording sheet is delivered between the sleeve and a recording electrode disposed in an opposed relation to the sleeve. When electric data signals are applied to the electrode, a latent image is formed on the recording sheet and simultaneously the magnetic toner is transferred to the recording sheet.
Regarding such direct recording methods, there has been many other proposals, all of which, however, are concerned with electrically recording methods using a recording electrode and a counter electrode opposed thereto. To obtain better recorded images, these methods must be carried out under a variety of strictly selected conditions including a gap between both electrodes, an amount of toner supplied to the recording sheet, etc. When a plain paper is used as the recording sheet, a surface potential of the paper is considerably affected by environmental conditions such as a moisture, a temperature and the like. Therefore, since developing parameters must be adjusted depending on the environmental conditions, the direct recording methods using plain papers as the recording sheet has not yet been reduced into practice.
On the other hand, there has been proposed a method in which an image light corresponding to an original image is illuminated from a back side of a light-transmittable image-bearing member to form an electrostatic latent image on a surface of the image-bearing member, and the latent image is developed simultaneously by selectively attracting thereon a magnetic conductive toner which is supplied by a developing roller composed of a permanent magnet and a sleeve. The developed image is then transferred and fixed onto a recording sheet. However, the above-mentioned recording method has adopted a corona discharger for imparting an electrostatic charge to the image-bearing member in a charging section and a transferring section. Since the corona discharger employs a metal wire applied with a high voltage such as D.C. 5-8 kV, undesired by-products including ozone, nitrogen oxides (NOx), etc. are generated upon corona discharging. This leads to air pollution such as generation of discomfortable odor, etc. The by-products produced due to the corona discharging might cause deterioration of the image-bearing member, obscure or deteriorated toner image, etc. The corona discharger has a large power consumption because only 5-30% of an electric current supplied thereto is utilizable for the charging. Further, if the metal wire of the corona discharger is stained, the toner images having low quality such as lost or lacked toner images, undesired toner lines, etc. are produced. In the transferring section, the corona discharger is disposed on a back side of the recording sheet and applies a charge having a different polarity from that of the image-bearing member to the developer through the recording sheet, thereby transferring the toner image to the recording sheet. As a result, the transferring process is considerably affected by an ambient moisture which causes change in an electrical resistance of the recording sheet. If the resistance of the recording sheet is low, the transferring of the toner images is undesirably prohibited.
Furthermore, there has been proposed a method in which the image-bearing member is electrostatically charged by brushing with a fur brush. The charging system has such a defect that residual toner and fogging toner are brushed together, resulting in contaminating the surface of the image-bearing member and therefore deterioration in quality of subsequently produced toner images.